Two kinds of STN type liquid crystal display devices having liquid crystal with a twist angle of 180.degree. or more have been commercialized so far: one with a 240.degree. twist angle and another with a 260.degree. twist angle. But recently better temperature characteristics have been demanded with liquid crystal display devices. To be more specific, there is demanded an improvement on a contrast change due to a temperature change and an improvement on luminence non-uniformity caused by heat emitted from a backlight in a modular liquid crystal display device. Therefore, of the two kinds of STN type liquid crystal display devices, the one with a 260.degree. twist angle is becoming more and more preferred to the other. This trend is a result of a fact: liquid crystal with a greater twist angle allows less affection by the temperature change on d.multidot..DELTA.n (i.e., a product of cell thickness `d` and a birefringence index of liquid crystal `.DELTA.n`) and consequently has a smaller contrast change. In short, liquid crystal with a greater twist angle can produce better quality in display.
Meanwhile, small pretilt angles of 2.degree. to 4.degree. are used for the STN type liquid crystal display device with the 240.degree. twist angle (hereafter referred to as the 240.degree. twist system). However, in order to regulate liquid crystal molecules' orientation on an orientation film surface, i.e., in order to firmly anchor liquid crystal molecules on an orientation film surface, a greater twist angle requires greater pretilt angles. Strength to regulate the liquid crystal molecules by the orientation film surface, i.e., anchoring of the liquid crystal molecules on the orientation film surface is made weak by the small pretilt angles. Therefore, liquid crystal molecules in an STN type liquid crystal display device having liquid crystal with a greater twist angle such as 260.degree. (hereafter referred to as the 260.degree. twist system) but with the same small pretilt angles of 2.degree. to 4.degree. may not respond to an applied electric field. Such an inadequate response of the liquid crystal molecules to the applied electric field causes inadequate display (in other words, the inadequate response causes the under twist domain to occur). Hence, firmer anchoring of the liquid crystal molecules and greater d/p margin are needed (`d` represents cell thickness and `p` represents spontaneous twist pitch).
Consequently, pretilt angles to two respective substrates of a conventional 260.degree. twist system have been set to 4.degree. to 7.degree. in order to anchor liquid crystal molecules by taking 4 parameters into consideration: anchoring of the liquid crystal molecules on orientation film surfaces, liquid crystal bistability, an occurrence rate of focal conic, and an occurrence rate of the under twist domain.
Theoretically, there are 3 methods to obtain great pretilt angles: (1) a change of liquid crystal composition, (2) a change of materials of an orientation film (e.g. a change of surface polarity or composition), and (3) a change of processing conditions of an orientation film (e.g., a change of baking conditions or post-baking processing conditions). But, practically no liquid crystal material can produce both great pretilt angles and basic necessary characteristics of liquid crystal such as Tni (a transition point), .DELTA.n (a birefringence index), V (driving voltage), and .alpha. (sensitivity). Therefore, the method(s) (2) and/or (3) are/is generally used in order to obtain great pretilt angles.
As described above, in a liquid crystal display device with a great twist angle, the anchoring of the orientation of the liquid crystal molecules on the orientation film surfaces is strengthened by the great pretilt angles of 4.degree. to 7.degree.. However, when a bed of the orientation film is provided with a color filter and etc., an irregular bed surface may cause unsmooth orientation film surface. Consequently, the great pretilt angles result in non-uniform pretilt angles to the substrate, thereby causing inadequate orientation of the liquid crystal molecules and low display quality. To be more specific, the irregular bed surface of the orientation film causes non-uniform pretilt angles on the whole substrate surface, as well as non-uniform printing and rubbing processes of the orientation film. The non-uniform pretilt angles cause non-uniform threshold voltage and display since the STN type liquid crystal display device utilizes, for display, high sensitivity of transmissivity characteristic to an applied voltage. In other words, non-uniform pretilt angles cause inadequate orientation of the liquid crystal molecules in accordance with correlation between an applied electric field and the anchoring of the liquid crystal molecules on the orientation film surface. Therefore, the non-uniform pretilt angles increase inadequate display besides inadequate display such as rubbing scratches and degrade display quality of the liquid crystal display device.
These problems can be solved by raising viscosity of the liquid crystal molecules to lower the sensitivity (hereafter referred to as .alpha. value), thereby increasing dependence of the liquid crystal molecules on an applied electric field rather than on a state of the bed surface. However, since this method worsens the rising characteristic of the liquid crystal molecules and reduces the contrast, here comes up another problem of difficulty in maintaining both the good optical characteristic and the good display quality.
It should be noted here that the problems only occur to an STN type liquid crystal display device with a twist angle of 250.degree. or more. On the contrary, an STN type liquid crystal display device with a twist angle of less than 250.degree. has no such problems since such a liquid crystal display device (1) does not need firm anchoring of the orientation of the liquid crystal molecules on the orientation film surfaces and (2) is arranged to have pretilt angles of 2.degree. to 4.degree. with the four parameters taken into consideration: anchoring of the liquid crystal molecules on the orientation film surfaces, liquid crystal bistability, an occurrence rate of focal conic, and an occurrence rate of the under twist domain. But, the STN type liquid crystal display device with the twist angle of less than 250.degree. is not discussed in the present application since it does not produce as good display quality as the STN type liquid crystal display device with the twist angle of 250.degree. or more.